STALL FIGHT TESTINGStall Characteristics Analysis
The analysis of stall characteristics is generally performed by inspection of conventional time histories of each stall conventional maneuver. The comments of the engineering flight test pilot and the certification test pilot are of utmost importance in the acceptably the stall characteristics.
There is such a wide variety of potential undesirable stall characteristics that only general discussions can be presented. Characteristics such as uncontrollable pitch-up, insufficient elevator to recover, uncontrollable roll-off anal unacceptably high structural buffet are typical concerns. Though not unacceptable, insufficient elevator to produce a full aerodynamic stall at a foward center of gravity position might not be acceptable relative to performance penalties.
Several sketches follow to illustrate what the analyst might expect to observe if one of these undesirable characteristics should occur.
ACCEPTABLE CHARACTERISTICS
For an airplane with good stall characteristics, a time history might be expected to look like that in Figure 1 the airplane pitches nose down while the pilot is still pulling back on the elevator indicating a full aerodynamic stall with the elevator travel matched closely to that needed for the Cg position of the test. The minimum speed is the stall speed.
UNCONTROLLED ROLL-OFF
If an uncontrolled roll-off should occur, as sketched in the time history below that indicates even with full corrective aileron, the airplane continued to roll. The point of full aileron as in Figure 2 with continued rolling would be considered as stall speed. This would penalize the airplane performance. Depending upon the hazardous and abruptness of at the rolling condition, the certification agency might require that the characteristic be eliminated before certification can be granted.
UNCONTROLLED PITCH-UP
If an uncontrolled pitch-up should occur, a time history as sketched in Figure 3 below would indicate that with full CORRECTIVE elevator, the airplane continued to pitch up The point of full nose-down elevator with continuing pitch-up would be considered as stall speed. This would also penalize the airplane performance. Depending upon the hazardous and abruptness of at the pitching condition, the certification agency might require that the characteristic. be eliminated before certification. can be granted.
ELEVATOR LIMITED STALL
Though not hazardous, some airplanes, at a foward center of gravity position have insufficient elevator power to drive the airplane to a full aerodynamic stall. This penalizes the airplane performance. If such a characteristic should occur, a time history as sketch below in Figure 4 would illustrate the characteristic.. Here, the elevator is against the nose-up stops and the airplane maintains a near constant airspeed. This effect is further illustrated in Figure 5 by the summary plot below where, as a function of center of gravity position a reduction in maximum lift coefficient is seen. This, when compared to what would be expected for a full aerodynamic stall, would indicate example level of lost performance and hence the amount of performance penalty.
Lessons
An air data system that is accurately calibrated to include stall and has known lag characteristics is fundamental to this test.
A definition of stall, either from a contractual standpoint or as agreed upon between the contractor and using test agency must be established. Minimum airspeed is used here for convenience. Most agencies require the flight manual to be built to the most critical conditions such as forward cg position on a conventional aft tail aircraft. Modern aircraft that may have canards, stick pushers, digital flight controls, be elevator limited, etc. may mask the speed variations in a non-aerodynamic form so be aware of the characteristics of each aircraft under evaluation.
In general, before beginning the stall with the aircraft in fixed conventional configuration, the pilot must trim the aircraft at some speed greater than Vs, i.e. for civil authorities between 1.2Vs and 1.4Vs.
A swept wing aircraft with fuel tanks that allow fuel to shift outboard and hence aft at these higher pitch angle conditions will usually create poorer stall characteristics that in turn might place unnecessary limitations on the stall limiter settings.
SUPPORT FACILITIES
Stalls may be critical and hazardous depending upon your situation. Generally, a safety chase aircraft capable of speed approximating the stalls speed of your test airplane will be necessary.
After the basic safety of the aircraft has been established, many flight test organizations use telemetry to monitor progress. data quality and safety.
Though not normally thought of as a facility, an air data system capable of measuring calibrated airspeeds and altitudes down to and including test stall speed is a necessary facility.
STALL SPEED DETERMINATION
The purpose of the test is to establish the maximum lift coefficient for each applicable aircraft configuration. This will provide information of both contractual guarantees and flight manual.
In general, with the aircraft in fixed trim. at a specified percentage of stall speed, usually between 1.2 and 1.4Vs, a constant entry rate stall maneuver is performed. As a minimum, the entry rate should be constant from 1.1Vs to the minimum speed. Working from a time history plot of airspeed, altitude, Mach number, and thrust parameter the values at stall speed are obtained plus the slope (entry rate) of the airspeed from 1.1Vs to stall. These numbers are used in the equation,
CLmax = Weight / (qe Sw)
where
qe = true dynamic pressure, psf
NOTE: qe notation is used here to minimize potential conflict in using qc derived from calibrated airspeed.
Sw = reference wing area, ft2
Because of large cg variations among data, it may have been desirable to correct to a Standard cg position which in turn should reduce data scatered This is been done by,
CL(std)) = CL(test) [1 -c/lt(cgtest-cgstd) ]
The next step is to establish the variation of stall with entry rate. In the simplest form, plots of CLmax vs entry rate are made with various symbols used to isolate variations in configurations of cg, thrust, flaps, etc. as illustrated here in Figure 6 . Note that the guide lines on Figure 6 were developed from the data by identifying variations with comparable configurations, etc.
There is a degradation of CLmax with Mach number. This traditionally is considered in the cruise or high speed configuration and not in the airfield flight configurations. Usually, this is developed from a plot of CLmax vs Mach number seen in Figure 7
An evaluation of the Mach existing at stall can be beneficial to the analysis even in the airfield configurations in that stall demonstrations are usually performed at much higher altitudes than that nominally flown in operations.
Development of the traditional flight manual stall speed chart is done by assuming an array of gross weights sufficient to define the operational range and then thorough a series of altitude assumptions for each weight, iterating on the Mach number at stall, the calibrated airspeed that would exist at stall is computed and presented like the illustration of Figure 8 .
In the development of stall speeds for a new airplane, detailed attention to the sequencing of the tests to define stall speed and configuration control is necessary. Many times, this is an iteration process which can be both time consuming and expensive. Finalization of hardware, even "under development" systems require official determination of the stall speeds. This is fundamental to further testing. All flying quality, performance and some structural test are predicated on the values of stall speed for the various configurations.
Generally, a series of stalls are performed beginning in a mid cg range; then progressing to forward and then aft on subsequent flights. The series begins at a light to medium gross weight and progresses up to maximum flight weight. With in-flight refueling this can be a significant impact to scheduling as maximum flight weight can be greater than takeoff weight in some cases.
If no undesirable characteristics are encountered, and no artificial system is necessary, a miracle has just occurred. Usually, some artificial stall limiter, stall barrier, stick pusher, etc. has to be added to the control system and set to accommodate an undesirable characteristic. Once the setting has been established, the series of stalls has to be repeated to assure the airplane cannot enter the undesirable regime. It cannot be over emphasized that airplane configuration control is essential. If a change to the limiter hardware system is made after the stall limiter demonstration portion, it is altogether possible that the whole series may have to be repeated again. This is where demonstration using a bread board or non-production system may cause additional stalls to be performed when the production version becomes available.
PROCEDURE:
The following is provided to illustrate the magnitude of a stall test program where many combinations of flight configurations and subsystem failures are possible.
Four to six of the following stalls should be performed at each condition, with stall limiter optimized and operative, to evaluate the airplane stall performance.
NOTE: Stall characteristics tests will be described in a separate section.
For the best performance stall data, the stall entry rate should be varied from 1.5 to 0.5 knots per second at each test condition specified and recovery initiated at stall limiter operation for each test. The airplane will be trimmed for "hands off" flight at the appropriate performance trim speed. If necessary, the airplane will be accelerated to a higher speed, with trim constant, so that the required steady entry rate can be established prior to encountering stall warning.
The effects of thrust variation will have been determined during the development testing. If these effects are shown to be appreciable, these tests will be repeated with thrust for level flight.
| TRIM ALTITUDE | WEIGHT | CG | |
| SPEED | |||
| 1.2 Vs | l0,000 | Heavy | Fwd |
| l.2Vs | 10,000 | Medium | Fwd |
| 1.2Vs | 10,000 | Light | Fwd |
| 1.2Vs | 10,000 | Heavy | Aft |
| 1.2Vs | 10,000 | Medium | Aft |
| 1.2Vs | 10,000 | Light | Aft |
The repeating of each test point with stall limiter "OFF" is a point to be negotiated within the Test Team. If the team does not wish to carry each run to aerodynamic stall, the stall limiter will be required to dispatch the airplane for each operational flight.
Stalls may be critical and hazardous depending upon your situation. Generally, a safety chase aircraft capable of speed approximating the stall speed of your test airplane will be necessary.
Though not normally thought of as a facility, an air data system capable of measuring calibrated airspeeds and altitudes down to and including test stall speed is a necessary facility.
An air data system that is accurately calibrated to include stall and has known lag, characteristics is fundamental to this test.
A definition of stall, either from a contractual standpoint or as agreed upon between the contractor and using test agency must be established. Minimum airspeed is used here for convenience. Most agencies require the flight manual to be built to the most critical conditions such as forward cg position on a conventional aft tail aircraft. Modern aircraft that may have canards, stick pushers, digital flight controls, be elevator limited, etc. may mask the speed variations in an aerodynamic form so be aware of the characteristics of each aircraft under evaluation.
In general, with the aircraft in fixed conventional trim the stall speed will improve when trim is at the lower speed range, i.e., 1.2Vs.
A swept wing aircraft with fuel tanks that allow fuel to shift outboard and hence aft at these higher pitch angle conditions will usually create poorer stall characteristics that in turn might place unnecessary limitations on the stall limiter settings.
STALL CHARACTERISTICS PROCEDURES
The following stalls will be performed with stall limiter optimized, if applicable and operative to evaluate the airplane stall characteristics. The airplane will be trimmed for "hands off" fight at 1.3 Vs, then decelerate the airplane to stick pusher, if applicable. The stall entry rate will be less than 1.0 knot per second and recovery will be initiated at pusher' operation for, each test point.
Repeat each test point with stall limiter "OFF". Carry each run to aerodynamic stalk Repeat each test paint as. necessary.
NOTE: Engine thrust "ON" refers to TFLF @1.6Vs in the approach configuration at maximum landing weight.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | UP | UP | ON | ON |
| 5000 | TOl | UP | IDLE | IDLE |
| 5000 | TO1 | UP | ON | ON |
| 5000 | TOl | DN | ON | ON |
| 5000 | TO2 | UP | IDLE | IDLE |
| 5000 | T02 | UP | ON | ON |
| 5000 | T02 | DN | ON | ON |
| 5000 | LDG | OP | IDLE | IDLE |
| 5000 | LDG | DN | ON | ON |
| 20000 | LDG | DN | IDLE | IDLE |
| 20000 | IDLE | DN | ON | ON |
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
| 23000 | TOl | UP | IDLE | IDLE |
| 23000 | TOl | UP | ON | ON |
| 23000 | T02 | UP | IDLE | IDLE |
| 23000 | T02 | UP | ON | ON |
| Max Alt | UP | UP | IDLE | IDLE |
| 5000 | T02 | ON | TO | TO |
NOTE:
Throughout this Stall Characteristic discussion, the notation TOl and T02 refer to the first and second takeoff flap setting. Generally, the contractor supplies the customer with several takeoff flap setting for takeoff performer field length optimization.
Perform the following approaches with the airplane trimmed at 1.3Vs, and in a 30 degree right bank Using elevator control only, decelerate the airplane to stick pusher or minimum speed, whichever occurs first.
Repeat these approaches in a 30 degree left bank.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | IDLE | UP | ON | ON |
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
| 5000 | LDG | DN | IDLE | IDLE |
| 5000 | LDG | DN | ON | ON |
| 20,000 | LDG | DN | IDLE | IDLE |
| 20,000 | LDG | DN | ON | ON |
The following stalls will be performed with stall limiter optimized and operative to evaluate the airplane stall characteristics. The airplane will be trimmed for "hands-off" flight at 1.3Vs, then decelerate the airplane to stick pusher, if applicable. The stall entry rate will be less than 1.0 knot per second and recovery will be initiated at pusher operation for each test point.
Perform these points with SPEED BRAKE EXTENDED if applicable.
Repeat each test point with stall limiter "OFF". Carry each run to aerodynamic stall.
Repeat each test point as necessary.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | ||
| FEET | LEFT RIGHT | ||||
| 23000 | UP | UP | IDLE | IDLE | |
| 27000 | UP | UP | ON | ON | |
| 45000 | UP | UP | IDLE | IDLE | |
| 5000 | TO1 | UP | IDLE | IDLE | |
| 5000 | TO2 | UP | IDLE | IDLE | |
HEAVY WEIGHT/AFT CG,
REFERENCE - FAR 25.103, 25.201,255.2O8
The following stalls will be performed with stall limiter optimized and operative to evaluate the airplane stall characteristics. The airplane will be trimmed far "hand off" flight at 1.3 VS, then decelerate the airplane to stick pusher if applicable. The stall entry rate will be less than 1.0 knot per second and recovery will be initiated at pusher operation for each test point.
Repeat each test point with stall limiter "OFF". Carry each run to aerodynamic stall. Repeat each test point as necessary.
NOTE:
Engine thrust "ON" refers to TFLF @1.6Vs in the approach configuration at maximum landing weight.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | UP | UP | ON | ON |
| TOl | UP | ON | ON | |
| 5000 | T02 | UP | ON | ON |
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
| 5000 | LDG | DN | IDLE | IDLE |
| 5000 | LDG | DN | ON | ON |
| 5000 | LDG | ON | IDLE | IDLE |
| 20000 | LDG | ON | ON | ON |
| 5000 | UP | UP | IDLE | IDLE |
Perform the following approaches with the airplane trimmed at 1.3Vs, and in a 30 degree right bank. Using elevator control only, decelerate the airplane to stick pusher or minimum speed, whichever occurs first.
Repeat these approaches in a 30 degree left bank
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
NOTE:
Repeat test points as necessary.
The following stalls will be performed with stall limiter optimized and operative to evaluate the airplane stall characteristics. The airplane will be trimmed far "hands off" flight at 1.3Vs, then decelerate the airplane to stick pusher. The stall entry rate will be greater than 1.0 knot per second but not more than 4.0 knots/second or 3.0 degrees pitch per second and recovery will the initiated at pusher operation for each test point.
Repeat each test paint as necessary.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | LDG | DN | IDLE | IDLE |
LIGHT WEIGHT/AFT CG
REFERENCE - FAR 25.103, 25.201, 25.208
The following stalls will be performed with stall limiter optimized and operative to evaluate the airplane stall characteristics. The airplane will be trimmed far "hands off" flight at 1.3 Vs,then decelerate the airplane to stick pusher. The stall entry rate will be less than l.0 knot per second and recovery will be initiated at pusher operation for each best point.
Repeat each test paint with stall limiter "OFF". Carry each run to aerodynamic stall. Repeat each test point as necessary.
NOTE:
Engine thrust "ON" refers to TFLF @1.6Vs in the approach configuration at maximum landing weight.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | UP | UP | ON | ON |
| 5000 | TOl | UP | IDLE | IDLE |
| 5000 | TOl | UP | ON | ON |
| 5000 | T02 | UP | IDLE | IDLE |
| 5000 | T02 | UP | ON | ON |
| 5000 | IDLE | DN | IDLE | IDLE |
| 5000 | IDLE | DN | ON | ON |
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST (Table continued) | |
| FEET | LEFT | RIGHT | ||
| 20000 | LDG | DN | IDLE | IDLE |
| 20000 | LDG | DN | ON | ON |
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
| 230O0 | TO1 | UP | IDLE | IDLE |
| 23000 | TO1 | UP | ON | ON |
| 23000 | TO2 | UP | IDLE | IDLE |
| 23000 | TO2 | UP | ON | ON |
| Max Alt | UP | UP | IDLE | IDLE |
Perform the following approaches with the airplane trimmed at 1.3Vs, and in a 30 degree right bank. Using elevator control only, decelerate the airplane to stick pusher or minimum speed, whichever occurs first.
Repeat these approaches in a 30 degree left bank.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | UP | UP | ON | ON |
| 5000 | IDLE | ON | IDLE | IDLE |
| 5000 | IDLE | ON | ON | ON |
| 20000 | LDG | DN | IDLE | IDLE |
| 20000 | LDG | DN | ON | ON |
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
NOTE:
Repeat test points as necessary.
The following stalls will be performed with stall limiter optimized and operative to evaluate the airplane stall characteristics. The airplane will be trimmed for "hands off" flight at 1.3Vs, then decelerate the airplane to stick pusher. The stall entry rate will be greater than 1.0 knot per second but not more than 4.0 knots/second or 3.0 degrees pitch per second and recovery will be initiated at pusher operation for each test point.
Repeat each test point as necessary.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | UP | UP | ON | ON |
| 5000 | TO1 | UP | IDLE | IDLE |
| 5000 | T02 | UP | IDLE | IDLE |
Repeat approaches as necessary to satisfy the test requirements
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | LDG | DN | IDLE | IDLE |
| 5000 | UP | UP | ON | ON |
| 5000 | UP | DN | ON | ON |
HEAVY WE1GHT/FORWARD CG, VARIABLE ENTRY RATE STALLS,With the airplane trimmed at 1 .3VS, and idle thrust, use elevator control to maintain a constant rate off deceleration ( 0.5 knots/second ) until minimum speed stall limiter activation, whichever occurs first.
Repeat at deceleration rates of 1.0 and 1.5 knots/second.
Perform a minimum of two approaches for each configuration.
Repeat as necessary to satisfy the test conditions.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | TO1 | UP | IDLE | IDLE |
| 5000 | T02 | UP | IDLE | IDLE |
| 5000 | LDG | DN | IDLE | IDLE |
| 15000 | UP | UP | IDLE | IDLE |
| 15000 | TOl | UP | IDLE | IDLE |
| 15000 | T02 | UP | IDLE | IDLE |
| 15000 | IDLE | UP | LDG | IDLE |
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 23000 | UP | UP | IDLE | IDLE |
| 23000 | TO1 | UP | IDLE | IDLE |
| 23000 | TO2 | UP | IDLE | IDLE |
| 30000 | UP | UP | IDLE | IDLE |
| Max Alt | UP | UP | IDLE | IDLE |
HEAVY WEIGHT/FORWARD CG, THRUST EFFECTS STALLS
REFERENCE - FAR 25.103, 25.201, 25.203, 25.207
Set the engines for TFLF at 1.6Vs, trim the airplane at 1.3Vs and use elevator control to maintain a constant 1.0 (nominal) knot/second rate of deceleration until minimum speed or stall limiter activating whichever occurs first.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST |
| FEET | LEFT RIGHT | ||
| 5000 | TO1 | UP | TFLF @ 1.6Vs |
| 5000 | LDG | DN | TFLF @ 1.6Vs |
HEAVY WEIGHT/FORWARD CG, SPEED BRAKE EFFECTS STALLS,
REFERENCE - FAR 25.103, 25.201, 25.208, 25.207
Turn the airplane at idle thrust (1.3Vs) with the flight spoilers systematically deployed, use elevator control to maintain a constant 1.0 (nominal) knot/second rate of deceleration until minimum speed or stall limiter activation, whichever occurs first.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | TO1 | UP | IDLE | IDLE |
| 5000 | T02 | UP | IDLE | IDLE |
LIGHT WEIGHT/FORWARD CG, VARIABLE ENTRY RATE STALLS
REFERENCE: - FAR 25.103, 25.201, 25.208, 25.207
With the airplane trimmed at 1.3Vs, and idle thrust, use elevator control to maintain a constant rate of deceleration (0.5 knots/second) until minimum speed or stall limiter activation, whichever occurs first.
Repeat at deceleration rates of 1.0 and 1.5 knots/second.
Perform a minimum of two approaches for each configuration.
Repeat as necessary to satisfy the test conditions.
| ALTITUDE | FLAPS | GEAR | ENGINE THRUST | |
| FEET | LEFT | RIGHT | ||
| 5000 | UP | UP | IDLE | IDLE |
| 5000 | TO1 | UP | IDLE | IDLE |
| 5000 | T02 | UP | IDLE | IDLE |
| 5000 | LDG | DN | IDLE | IDLE |
| 15000 | UP | UP | IDLE | IDLE |
| 15000 | TO1 | UP | IDLE | IDLE |
| 15000 | T02 | UP | IDLE | IDLE |
| 15000 | IDLE | UP | IDLE | IDLE |